how to repeat an object on a path?
You should move your Array Modifier above your Curve Modifier, however, you also have the wrong option selected in the Array Modifier.

Your Array Modifier is set to "Object Offset" to calculate the array copies. As the curve and your array object share an origin, this makes no sense.

You need to untick "Object Offset" and tick "Relative offset" instead.

Then you can either manually set the number of array objects that you need or change "Fit Type" to fit to the length of the curve (a new box will appear allowing you to select your curve).